Python 3: fix lambda syntax

gracedb/superevents/views.py

@@ -233,7 +233,7 @@ class SupereventPublic(DisplayFarMixin, ListView):
                     ("BBH", voe.prob_bbh),
                     ("Terrestrial", voe.prob_terrestrial),
                     ("MassGap", voe.prob_mass_gap)]
-                pastro_values.sort(reverse=True, key=lambda (a,b): b)
+                pastro_values.sort(reverse=True, key=lambda a, b: b)
                 sourcelist = []
                 for key, value in pastro_values:
                     if value > 0.01: